Chemical Degradation Assessment of Fuel Cell Electrode Ionomer Using a Pellet-Form Catalyst Layer Analogue

Abstract

Electrode efficiency and durability remain key challenges for the adoption of proton exchange membrane fuel cells (PEMFCs). The ionomer, a material used for membranes as well as the binder in the catalyst layers, influences the electrochemical performance and lifetime of the cell. Chemical degradation of bulk perfluorinated sulfonic acid (PFSA) ionomer has been extensively studied, and the mechanisms of attack by radical species are well established. Numerous accelerated tests have been developed to test ionomers; a popular ex-situ chemical treatment is immersion in Fenton’s reagent, a mix of aqueous hydrogen peroxide and ferrous ions that generates hydroxyl radicals. However, few studies have used Fenton’s reagent in experiments targeting the electrode ionomer. Here, we report a method for forming centimeter-sized pellets of ionomer-catalyst composites using a procedure analogous to that used for PEMFC electrodes. These pellets comprise sufficient ionomer mass for fluoride measurement by ion-selective electrode (ISE) after Fenton tests, enabling degradation rate evaluation of the ionomer network structure found in catalyst layers. We present the fluorine loss rates of these pellets at various hydrogen peroxide concentrations, both with and without platinum. We also compare the pellets’ degradation with a membrane over a 48-hour test, showing an order-of-magnitude lower cumulative loss for the pellets when normalized by ionomer mass. From these results, we suggest that the structure of the catalyst layer may reduce ionomer degradation rates and that platinum may have an additional protective effect in Fenton tests. In addition, the pellet-based approach may be useful for other chemical analytical methods where the low mass of ionomer in electrodes results in signal below detection limits.